The usual way of operating pressure leaf filters employing flexible diaphragms for compressing the cakes on the leaves as disclosed in my said patents is to pressurize the filter chamber with gas to hold the cakes under compression between the diaphragms and the adjacent faces of the filter leaves during the time that the heel flows out of the chamber through a drain port at the bottom. The heel is recycled into the system where it is mixed with the prefilt for later filtration. After the liquid has been drained from the tankchamber pressure is returned to atmospheric to permit the resilient diaphragms to return to their relaxed positions away from the cakes which remain on the faces of the filter leaves. Thereafter the leaves are vibrated to cause the relatively dry filter cakes to separate from the leaves and to drop into a suitable receptacle or the like.
During the time that the filter cakes are being compressed by the diaphragms, the peripheral portions of the cakes continue to build up inasmuch as they are not covered by the diaphragms and liquid flows therethrough. Since these peripheral portions of the cakes are not compressed they have a much greater moisture content than does the remainder of the cakes. In many applications the solids content of this peripheral cake may be no more that about twenty-five percent of the solids content of the compressed part of the cake. While the heel is being removed from the filter chamber the lower part of this relatively wet, peripheral cake drops off and passes out through the drain port with the heel. However, the upper part of this peripheral cake remains on the main cake between the peripheral portions of the leaves and diaphragms after the heel has been removed. The remaining peripheral cake is undesirable for two principal reasons. One, it increases the moisture content of the overall filter cake which is later removed from the filter leaves during the vibration step, and two, it hinders the release of the diaphragms from the cakes.